Metal containers for foods and beverages are generally made from aluminum, tin-free steel, blackplate and tinplate steel (which is cold rolled steel sheet coated with a thin layer of tin). Because such metal containers are subject to air oxidation or rusting, and to corrosion caused during the pasteurization and sterilization of the beer, soda pop, and various food products which are packaged in such containers, coating compositions are used to protect the metal from chemical attack and to protect the food products from the deleterious byproducts of chemical attack. Due to their potential contact with food products, such coatings present additional problems of toxicology and taste considerations.
Investigators continually have sought improved coating compositions that reduce or eliminate corrosion of a metal article and that do not adversely affect an aqueous product packaged in the metal article. For example, investigators have sought to improve the imperviousness of the coating in order to prevent corrosion-causing ions, oxygen molecules and water molecules from contacting and interacting with a metal substrate. Imperviousness can be improved by providing a thicker, more flexible and more adhesive coating, but often, improving one particular advantageous property is achieved at the expense of another advantageous property. For example, if the adhesive properties of a coating are improved, the flexibility of the coating can be adversely affected.
In addition, practical considerations limit the thickness, adhesive properties and flexibility of a coating applied to a metal substrate. For example, thick coatings are expensive, require a longer cure time, can be esthetically unpleasing and can adversely affect the process of stamping and molding the coated metal substrate into a useful metal article. Similarly, the coating should be sufficiently flexible such that the continuity of the coating is not destroyed during stamping and molding of the metal substrate into the desired shape of the metal article.
Investigators also have sought coatings that possess chemical resistance in addition to corrosion inhibition. In addition, investigators are confronted with the problem of providing a coating that meets FDA regulations and that is stable and easy to apply. A useful coating for the interior of a metal container must be able to withstand the solvating properties of a product packaged in the metal container. The coating also must be able to withstand acidic products such as mayonnaise, meats, fish and pickles. Tuna packed in water or in oil are products that require a coating composition to impart both chemical resistance and corrosion inhibition. If the coating does not possess sufficient chemical resistance, components of the coating can be extracted into the packaged product and adversely affect the product. Even small amounts of extracted coating components can adversely affect sensitive products by imparting an off-taste to the product.
In addition, the coating must be able to withstand processing steps used in preparing the finished packaged product. For example, when foods such as soup are packaged in a container, the filled container is subjected to a sterilizing treatment during which the container is subjected to retort water (i.e., pressure cooker-type) or steam over a prescribed period (e.g., about 10 minutes to many hours). The coating composition must be able to withstand this treatment, and also must be capable of withstanding the forming and bending operations used in fabricating a metal can. Still further, the coating composition should be economical, and capable of being readily applied to metal surfaces.
Epoxy-based coatings and polyvinyl chloride-based coatings have been used to coat metal containers and closures for foods and beverages because these coatings exhibit an acceptable combination of adhesion to a metal substrate, flexibility, chemical resistance and corrosion inhibition. However, epoxy-based coatings have the serious disadvantage of being considerably more expensive than polyvinyl chloride-based compositions.
Coatings based on polyvinyl chloride or related halide-containing vinyl polymers, like polyvinylidene chloride, possess the above-listed advantageous properties of chemical resistance and corrosion inhibition, and also are economical. Nevertheless, investigators continually seek improved coating compositions for food and beverage containers and closures, and for metal containers in general, that retain the advantageous properties of adhesion, flexibility, chemical resistance and corrosion inhibition, and that are economical and do not adversely affect the taste or other esthetic properties of sensitive food and beverages packaged in the container. The present invention is directed to an improved polyvinyl chloride-based coating composition useful for coating the interior or exterior of metal articles, like can ends, can bodies or closures wherein the metal article can be a container for a food or beverage, or for a non-food product, such as a cleaner.
The present coating compositions obviate a primer coating for the metal substrate, and have an extensive range of uses. However, the present coating compositions can be used with a primer coating. The coating compositions of the present invention have excellent adherence properties with respect to unprimed and primed metal surfaces, and at the same time have outstanding chemical resistance properties. The present compositions also can be stored in a single unit package from manufacture until use without generating an unacceptable increase in viscosity.
Prior investigators have studied coating compositions including a combination of an epoxy resin, a solution grade vinyl chloride copolymer and a vinyl chloride dispersion resin. Kaufman, Jr. et al. U.S. Pat. No. 3,114,725 discloses a coating composition including the three above-listed ingredients. The compositions disclosed by Kaufman, Jr. et al. include 10 to 50 per cent by weight of the vinyl chloride dispersion resin. The coating compositions disclosed by Kaufman, Jr. et al. typically were packaged and stored as a two-unit system because admixture of all composition ingredients could lead to an unacceptable increase in viscosity within 24 to 48 hours. A careful selection of composition ingredients may provide a stable one-unit system. Bromstead U.S. Pat. No. 3,305,602 discloses a coating composition including a vinyl chloride dispersion resin, a solution grade vinyl chloride copolymer, and an acrylic resin.
Although the above-identified patents disclose compositions for coating a food or beverage container, the references do not teach or suggest a coating composition: (1) having 60% to 90% by weight of nonvolatile material of a vinyl chloride dispersion resin and (2) that can be stored as a single unit system for an extended time period, which, during and after curing, demonstrates: (1) excellent flexibility; (2) excellent adhesion; (3) excellent mar and scratch resistance; (4) excellent chemical resistance and corrosion inhibition; and (5) improved economics compared to standard polyvinyl chloride coatings.
As an added advantage, the present coating compositions also can be used both on the interior can ends and on can bodies, obviating the use of two different coating compositions by container manufacturers. Furthermore, the present coating compositions exhibit sufficient clarity, hardness and mar resistance after curing for use as a coating on the exterior of a metal container. Accordingly, because of improved chemical and physical properties and improved economics, a coating composition of the present invention has a more universal range of applications, such as for the interior coating of a metal container or closure for food or beverage products or for the exterior coating of a metal container; and overcomes the economic disadvantages presented by prior epoxy-based and polyvinyl chloride-based coatings.